More 2H compression numbers

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Radd Cruisers said:
The earlier H and 2H were sleeved, and the later ones were parent bore.

The compression numbers that are in the upper 300 range are on the low side of things.

If the numbers are changing a lot hot and cold - check the valve clearances. There will be changes due to temperature....

Make sure that on VSV shutdown engines that the choke plate is open while testing (it should be by default).
On diaphragm fuel controlled engines, make sure that the throttle plate is wide open.

(note that I didn't read the thread, just the last couple of posts).

~John
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Thanks John. If you would, please chime in on the numbers posted up in this thread, starting from the top. And can you verify the accuracy of the FSM (398 psi being factory spec for a 2H, 30-35 PSI being the max healthy differential)?
 
Tofudebeest said:
The FSM is right. How could a compressed (that's what a turbo is, and why the MB turbo is called a "kompressor") engine have (factory) compression values LOWER than its nearly identical naturally aspirated predecessor? After all, the primary difference between those motors is merely the piston skirt cooling, and of course the turbo. Same bore and stroke, same displacement. Look at the compression ratios...

The 2H is a 4.0 L (3980 cc) inline 6, 12 valve OHV diesel engine. Bore is 91 mm and stroke is 102 mm, with a compression ratio of 20.7:1. Output is 103 hp (77 kW) at 3500 rpm - later production years 107 hp (80 kW) with 177 ft·lbf (215 N·m) of torque at 2000 rpm.


The 12H-T is a 4.0 L (3980 cc) inline 6, 12 valve OHV turbocharged diesel engine. Bore is 91 mm and stroke is 102 mm, with a compression ratio of 18.6:1. Output is 135 hp (101 kW) at 3500 rpm with 231 ft·lbf (312 N·m of torque at 2000 rpm.

I think somebody's compression readings are in error.
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Well, it's entirely possible that I am wrong on all three readings using the same gauge, if the gauge is faulty or if I frigged up the testing procedure. If this is true, I would sure like to know what the hell I'm doing wrong.
 
kim said:
I certainly wont complain about the numbers!

However, I do wonder about the 398 psi number in the 1985 FSM.
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Actually my FSM dates of 1980. It was reprinted in '85 but the specs in it are based on the August 1980 production model.

Also, it states a max difference between cylinders of 28 PSI, not 2,8 as I read in one of the posts here.

I'm pretty sure it's just the gauge you are using. When I had mine checked, the mechanic switched gauge half way thru and BOOM, numbers instantly got 40 PSI higher... Not the most accurate of sciences...

:cheers:
 
according to my engine manual 2H should give 398psi or more, 12H-T 427psi or more while differences between cylinders should be under 28psi...
 
Well, this is interesting enough that I just ordered an 86-90 2H engine manual from SOR, as one isn't available on buchanan and we are all referencing numbers for the 80-85 2H.
 
Tofudebeest said:
The FSM is right. How could a compressed (that's what a turbo is, and why the MB turbo is called a "kompressor") engine have (factory) compression values LOWER than its nearly identical naturally aspirated predecessor? After all, the primary difference between those motors is merely the piston skirt cooling, and of course the turbo. Same bore and stroke, same displacement. Look at the compression ratios...

The 2H is a 4.0 L (3980 cc) inline 6, 12 valve OHV diesel engine. Bore is 91 mm and stroke is 102 mm, with a compression ratio of 20.7:1. Output is 103 hp (77 kW) at 3500 rpm - later production years 107 hp (80 kW) with 177 ft·lbf (215 N·m) of torque at 2000 rpm.


The 12H-T is a 4.0 L (3980 cc) inline 6, 12 valve OHV turbocharged diesel engine. Bore is 91 mm and stroke is 102 mm, with a compression ratio of 18.6:1. Output is 135 hp (101 kW) at 3500 rpm with 231 ft·lbf (312 N·m of torque at 2000 rpm.

I think somebody's compression readings are in error.
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Tofu, I've been stewing on this post for a while now and one thing is bugging me and that is, how can the 2H have lower compression than the 12HT but have a HIGHER compression ratio?

My understanding is that the higher the compression ratio, the higher the compression. Now, if this is a fact, than there is something wrong in the 1985 FSM.
 
Tofudebeest said:
I know the later 2H was sleeved and had a stiffer block, but I can't see how the compression numbers would be different. John at Radd would know...

John? Can you clear things up?
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It was the early 2H motor that had sleeves[wet sleeve motor] and the later motor was not sleeved so therefore it had a stiffer block. It also had a revised oil cooler on the side of the motor and a different glow system.

Also the 2H and the 12HT motors have totaly different shaped combustion chamber and head design one being indirect with precup and the other direct injection, no glowplugs with different style of injector.
The piston in the 2H and 12Ht are totally different and are not interchangeable. They can fit in each bore but that is where it ends I think from memory the 12HT conrod is heavier and has a larger connection to the piston..

The listed figure for the compression is usually the lowest new motor reading at normal operating temperature.Importantly with the motor cranking at 250RPM or higher. Some of the old 2H starter motors crank well below this. Just listen to the sound of different 2H starters some are dry and have a dull crank LOL have lost the ZZZINg so to speak. I had to rebuild my starter motor as it was slow to start the motor sounding like it was working from a half flat battery and mates saying "you need to change your glow plugs"

If the head has been shaved ;[trued up] it will certainly make the compression higher as will carbon in the combustion chamber similarly sticky or badly adjusted valves will lower it apart from worn rings. cheers
 
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Folks. There is a difference between the compression ratios and therefore compression specs on the IDI (3B, 2H, 1HZ) and their DI brethern (13BT, 12HT, 1HD-T).

The FSM for the 3B/13BT and 1HZ/1HD-T both list the DI at a lower compression number. This makes sense. The compression ratio is lower in the DI version engines, as Kim has mentioned. bbd has covered why.

Now take the the 2H/12HT FSM. It lists the equivalent numbers as reversed. Makes total sense it is a typo.

We played with compression gauges a while back on one engine, as some different reading anomalies were noticed between two shops. The two gauges we had (ours and another shops) read approx 100psi different on the same engine...two complete runs down the block with each gauge, doen with-in minutes of each other. What was consistent between the two sets of numbers between the gauges was any difference between cylinders.
 
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Thanks for the input, Greg.

So, the 2H compression figure should be 427psi. I feel better now.

I guess now I need to calibrate my gauge somehow, find out if it is lying to me. If it is, hopefully it is lying consistently. :)
 
Greg_B said:
Folks. There is a difference between the compression ratios and therefore compression specs on the IDI (3B, 2H, 1HZ) and their DI brethern (13BT, 12HT, 1HD-T).

The FSM for the 3B/13BT and 1HZ/1HD-T both list the DI at a lower compression number. This makes sense. The compression ratio is lower in the DI version engines, as Kim has mentioned. bbd has covered why.

Now take the the 2H/12HT FSM. It lists the equivalent numbers as reversed. Makes total sense it is a typo.

We played with compression gauges a while back on one engine, as some different reading anomalies were noticed between two shops. The two gauges we had (ours and another shops) read approx 100psi different on the same engine...two complete runs down the block with each gauge, doen with-in minutes of each other. What was consistent between the two sets of numbers between the gauges was any difference between cylinders.
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CheerGB, I think this was thrahed out here about 2 to 3 years ago. There have been 3 or 4 things that have been printed wrong in the workshop manual.
 
Reviving a bit of an old post here, but this thread seems to be a good basis for a 2H comprression number reference.

I just got my HJ47 compression tested, and here's the numbers I got (in PSI):

No. 1: 480
No. 2: 470
No. 3: 480
No. 4: 440
No. 5: 450
No. 6: 470

Maximum difference between cylinders is 40psi - 12psi greater than the allowable stated in the FSM - BUT, only 8.3% difference - which is ok from what I understand...

So, according to my 2H FSM (1980), which most here have cited, these figures are way high ...... however, they tend to agree closely to the figures attained by both of Kim's 2Hs and CDN_Cruzer's 2H.
This indicates, to me, that no ones gauge is out of calibration, but that these engines have either:

a) higher compression values from standard than the FSM indicates

OR

b) these engines have some how increased in compression during their lifespan - maybe due to machining of the head like bigbrowndog mentioned. My engine had a rebuild about 160,000km ago, so most likely had some machining done on the head.

Opinions?.........

Cheers,

Matt
 
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